Run time error in Simulink model

I have designed ZigBee transmitter and reciever model on Simulink and when I merge bothe model using AWGN channel and started simulating it gets stuck at 99% and after that nothing is happening I am running the model for 20us.Someone please help me with this.

[Walter Roberson]

To confirm, this is a model of the entire process, rather than a model in which you are connecting to an actual ZigBee ?

If it is a model of the entire process then consider attaching the model for us to examine. (I do not have a ZigBee.)